Hi, Trina!
I love early planning!
This will help you get exactly what you want in most of your decisions, Trina, so excellent choice!
We've often traveled to Walt Disney World mid-March, and the weather is usually very mild. While there can be a period that's cooler than usual (some days were 50 degrees when we were there the last week of March this year) it can also get a bit warmer, too. (Average temperatures in March are 78 as a high, 54 as a low, according to Weather.com.)
Crowd-wise, touringplans.com (a great crowd-predicting site) projects the dates you mention to have average crowd levels of 7-9 out of 10 each day, but even on those days listed as "9" you can find a park with a much lower level of, say, 6.5, and simply choose that park that day.
Regarding resorts, take a look at what you get at a Value, Moderate or Deluxe Resort, as well as a Deluxe Villa. Knowing what you'd like in your room, as well as proximity to certain parks is a great first step. For example, would you like a mini-kitchen in your room? This narrows your options down to family suites at select Value and Moderate resorts or a studio in a Deluxe Villa. By exploring
resort hotels at Walt Disney World, you'll have a much better idea of your options and can begin to narrow them down from there.
Send more questions as you plan, Trina! I'd love to help make your 24th birthday and your son's 3rd birthday the most magical they can be!